THREE-DIMENSIONAL MODELING OF
THE DRIVING MECHANISM OF
THE 1976 TANGSHAN EARTHQUAKE IN CHINA

Loo, H.-Y., Gao, W.-A. and Yuan, Y.-G., 1989. Three dimensional modeling of the driving mechanism of the 1976 Tangshan earthquake in China. Journal of Geodynamics, 11:243-265.

A three-dimensional creep model of the driving mechanisms of the 1976 Tangshan earthquake is presented, with consideration of both the body force and the regional tectonic force. The upper crust is assumed to be a visco-elastic body, the strain rate for the viscous component is assumed to be linearly proportional to the deviatoric stress, but the lower crust and the upper mantle respond to the stress as a non-linear Newtonian fluid obeying the power law of creep. This model is solved numerically using the principle of additively of elastic, plastic and viscous strains, as well as the initial strain method with a tangential modulus for the upper crust and with a secant modulus for both the lower crust and the upper mantle to represent the energy dissipation of creep flow.

Computed results show that: 1) to match roughly the pre-seismic ground surface leveling and the co-seismic dip- and strike-slip, uplifting beneath the doming zone of the crust-mantle boundary plays a more important role than a horizontal driving mechanism; 2) the lower-crust fault first relaxes the strain induced by the uplift, resulting in stress accumulation in the upper crust, expecially at the front of the tip of the Tangshan fault lying above the crustal low-velocity zone; 3) significant co-seismic and post- seismic ground-surface displacements along the newly faulted belt in Tangshan result from a combina- tion of elastic strain rebound and change in gravitational potential.

INTRODUCTION

According to Huang (1980), stratigraphic, structural and seismic evidence indicates that cataclastic flow has been dominant in China since the late Paleozoic, and the present-day mechanical fabric of China was formed as a result of the Permo-Triassic and Cenozoic plate-tectonic collapse; there are six stable and rigid basement elements in China; and sediment patterns over the rigid basement blocks are uniform and lack sediment structures with steeply dipping faults at shallow depth; deformed ductile zones are made of up fragments; the regions between the rigid blocks and ductile zones are mechanically similar to the ductile zones.

Cenozoic plate motions caused continental collision in south-western China; however, only grabens formed in eastern China where island arcs moved away from the mainland. The driving mechanism for this graben formation is still uncertain. Both brittle elements and ductile zones have been rifted and cut by strike-slip faults in eastern China (Ma, 1984).

Basement behaviour in China can be characterized as follows: 1) where earthquakes occur, the behaviour is brittle; 2)where earthquakes do not occur, and there is evidence of strong deformation, the behaviour is purely ductile; and 3) aseismic regions such as basins surrounded be heavily defor- med zones are subject to tensional pulling and behave rigidly. For instance, in north China, most block-faulting basins are surrounded by earthquake- generating zones but no large shocks have occurred within the basins. This means that major shocks are associated with faults surrounding rigid blocks (Loo, 1980).

Tectonic features in north China cannot be described by a simple tectonic rigid model. From the macroscopic point of view, the regional tectonic force of the north China margin can be characterized by relative right-lateral motion between the Pacific plate and the continental plate (Ma, 1984). However, this is not sufficient to explain the nature of basin and range fault movements in terms of strike-slip with a dipping component and recent earthquake mechanisms in terms of strike-slip with normal components around the marginal basin. Therefore, the intracontinental phenomenon may not be directly related to subduction of the Pacific plate, it is most likely a manifestation of deep-seated processes on the surface (Loo et al., 1983; Mei, 1982 and Scalter and Christie, 1980). The heat-flow values and temperature distributions in the north China plain indicate upper-mantle upwelling as shown in Figs. 1 and 2.

As the lithosphere can flow over long periods of time in the geological time scale, visco-plastic creep of the lower part of the continental crust may be an important mechanism in the formation of the marginal basins and the inland stress can cause strain migration and/or stress accumulation in the relatively stiff upper crust. Geophysical and geological data collected on the ground surface need to be used to estimate a deep-seated process quan- titatively (Loo et al., 1982). In this paper, data on ground deformation, the relative gravity survey, the previously calculated crustal temperature field (Loo et al., 1983) and the creep test results (Wang, 1988; Yuen, 1978) of the crust and mantle rocks are combined to model lithospheric creep deforma- tion. Further, we illustrate a driving mechanism for the 1976 Tangshan strike-dipping earthquake (M = 7.8) determined by near-field body waves (Mi, 1982), which is different from the strike-slip with a thrust co-shock mechanism determined by teleseismic data (Butler et al., 1979).

In the north China plain, leveling information is recognized as a poten- tially valuable source of intra-continental crustal dynamics, and regional and local leveling results have recently been used to study intraplate defor- mation and the driving mechanisms of seismicity. Integrating other geophysical and geological information with leveling results is essential for proper interpretation of seismicities because the time-dependent ground deformation is a combined phenomenon of regional isostatic adjustment (Fig. 4), localized seismic fault movement (Fig. 5) and epicenter deforma- tion (Fig. 3). The gravity should also be taken into account for relatively realistic modeling of continental tectonics. For instance, the relationship between relative gravity, leveling data and variation of water level in a deep well (Fig. 3) can be attributed to uplifting at depth causing anomalous positive gravity values, ground surface doming and upper crustal dilation associated with the lowering of the water level.

In the Tangshan area, shallow faults overlying the low-velocity zone determined by both seismic and magnetotelluric soundings, have discon- tinuous geological features consisting of numerous discrete segments as shown in Fig. 6, they play an important role in the evaluation of principal aftershocks. In addition, there are several lower crustal faults underling the same low-velocity zone; they were detected by seismic sounding (Mei, 1982). During the deep-seated processes, the upper-mantle material moving up through the lithosphere would tend to follow the path of these deep faults, meeting with the least resistance (Mei, 1982).